There are no doubts that Facebook is the best social networking site in the world. It includes a huge number of users and it has provided a modern platform to keep your contact with friends and other relatives worldwide. It can be called as an era of Facebook fever.
I think it is the best social media platform going now, however I believe Twitter is a much more effective marketing platform for advertisements. Twitter outperforms Facebook (5 to 1)
Facebook is dying. The only growing segments of user is those 50+. The 20's to 30's crowd have moved onto easier to use (and less invasive platforms) like twitter or professional networks like Linkedin. The younger than 20's all are so used to text messaging and social media that the next big thing will cause them to shift (just look at how dominate myspace was ten years ago). No one wants a social network where your grandparents and parents can post your baby pictures. I mean it's sort of cute, but no one wants to look like an asshole in front of work contacts or clients, thus the surge to Linkedin and other less invasive social networks. Just my two cents, though.
I'm pointing out trends for the future. It's a fast changing market and things that may have worked in the past will not predict future performance. If Facebook can't get a handle on why people are fleeing, can't get a handle on invasion of privacy, and constantly changing formats that piss users off, they're going to die a dinosaur. They need to adapt and work on user retention. Otherwise, it's going to turn into a shell website where people have accounts but never use them.